Cationic catalysts are normally employed for effecting polymerization of epoxy compositions. A primary disadvantage of such systems resides in the fact that such photoinitiators are poisoned by bases, thereby retarding reaction or arresting it completely, particularly in the presence of high humidity and water.
In contrast, free radical reactions are highly desirable in adhesives, potting compounds and the like, for a number of reasons. They are capable of effecting deep curing (orders of magnitude thicker than similar resins cured with cationic photoinitiators), photocuring is typically completed in very brief exposure times, and there is little or no tendency for premature reaction.
The objects of the present invention are to provide compositions having the foregoing features and advantages.
It has now been found that certain of the foregoing and related objects of the invention are attained by the provision of a photopolymerizable composition comprising, based upon the weight of the composition, about 5 to 45 percent of an epoxy resin, about 94 to 55 percent of a copolymerizable material (i.e., monomer and/or oligomer), and about 1 to 10 percent of a free radical photoinitiator, the composition being devoid of any catalytic cationic ingredient. Comparable objects may be attained by the provision of a photopolymerizable composition comprising, based upon the weight of the composition, about 5 to 45 percent of an epoxy resin, about 94 to 55 percent of a copolymerizable material, and about 1 to 10 percent of a free radical photoinitiator, wherein the copolymerizable material includes an effective amount of at least one compound selected from the comonomer group consisting of vinyl and (meth)acrylic monomers containing acrylamide or amide functionality, or an hydroxyl group, and may or may not include a cationic catalyst. Typically, the members of the comonomer group employed will include N,N-dimethyl acrylamide, n-vinyl 2-pyrrolidone, n-vinylcaprolactam, acryloyl morpholine, N-(n-butoxymethyl) acrylamide, N-isopropyl acrylamide, N-3 dimethylaminopropyl methacrylamide, glycerol 1,3-diglycerolate diacrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, and (meth)acrylic acid; this group may also include acrylated polyols and vinyl polyols, albeit such compounds may be of either monomeric or oligomeric character.
When the comonomer is or includes a compound containing the amide or acrylamide functionality, that compound will usually be present in an amount not in excess of 80 weight percent, and preferably the amount will be at least about 50 weight percent. When the comonomer is or includes a compound containing the hydroxyl group, that compound will usually be present in an amount not in excess of about 70 weight percent, and preferably the amount will be at least about 20 weight percent. The composition may, more specifically, comprise about 15 to 30 weight percent of epoxy resin, about 40 to 60 weight percent of at least one comonomer, about 20 to 35 weight percent of an oligomer that is reactive with the epoxy resin and the comonomer, and about 2 to 5 weight percent of the photoinitiator.
Additional objects of the invention are attained by the provision of a fluid mixture capable of curing to a substantially nonshrinking and immobile solid mass (e.g., a positioning resin), comprising: about 10 to 50 percent, based upon the weight of the mixture, of a photopolymerizable composition, as herein described, which is capable of reaction to form a solid, resinous matrix; and conversely, about 90 to 50 percent, based upon the weight of the mixture, of a solid filler comprised of spherical elements and short fibrous elements present in a spherical element:fibrous element weight ratio in the range 0.1 to 6:1, the filler elements being substantially nonreactive to the reactive composition and exhibiting good adhesion to the resinous matrix. Preferably, the amount of reactive composition will be at least 25 weight percent, and the amount of the filler elements will not exceed, in combined weight, about 75 percent.
The spherical filler elements will usually constitute about 30 to 45 weight percent of such a mixture, and the fibrous elements may constitute about 10 to 80 weight percent thereof. Preferably, the fibrous elements will constitute about 20 to 30 weight percent of the mixture, and the spherical element: fibrous element ratio range will be about 1 to 3:1. The spherical and fibrous filler elements will generally be made of glass, for transparency and low coefficient of thermal expansion (albeit ceramic, mineral, metal, and synthetic and natural resinous elements may be employed in certain instances), and they may or may not carry a size coating. The spherical elements will preferably be hollow, with a distribution range of 5 to 20 microns and a mean size of 9 to 13 microns; the fibrous elements will preferably be short, small diameter milled fibers with a screen size (hole diameter) parameter ranging from {fraction (1/16)} to {fraction (1/64)} inch. Typically, the sphere diameter will be 10 mils or smaller, and equal to or (preferably) less than the diameter of the fibers. The size, composition, and form of the spheres and fibers will generally be selected so as to afford good suspension stability in the mixture. The solid mass produced will most desirably have a glass transition temperature in excess of 100xc2x0 C.